Present day society relies on extensive use of automation for packaging products for sale. This usually entails a means for determining the quantity of material placed within a retail package. Weighing is the most practical and acceptable form for controlling the material placed in a package and this is accomplished largely by scales known as in-motion weighing devices or checkweighers. Because of the speeds involved, this requires very precise real time measurement of containers prior to entering the weighing/filling station and during the filling operation so that filling may be terminated at a precise point. Setting up such a system is critical, the slightest miscalculation will result in enormous losses by over filling packages or excessive customer complaints and lawsuits due to under-filled packages.
In the past, the very critical operation of setting up an in-motion weigher was usually accomplished manually with the aid of an assortment of calculating devices and test equipment. The steps included a first operation where the load of the platform sitting on top of the transducer device was calculated. This is commonly referred to as the dead load.
Once the dead load was established, a ranging function was set. This was accomplished via a potentiometer using the highest weight package to set up the system. Alternately, the gain or range potentiometer was adjusted each time a different weight range was used.
Typical transducers used in such systems are a strain gauge load cell or a flexure type scale system using either a LVDT (Linear Variable Differential Transformer), a DCDT (Direct Current Differential Transducer), or force restoration (a magnetic means of counteracting forces in the scale by pumping current in the opposite direction to bring the scale back to an equilibrium point which must be offset).
Because the systems are electronic, they have some inherent noise imposed on control signals; typically, in-motion or checkweighing scales have noise in the range of 20 Hz or lower. This and higher values of noise have been filtered out by a low pass filter. However, the method by which this was done in the past used typical active analog filters and required quite a bit of human intervention to monitor the actual scale output and determine the appropriate resistor network or resistor values to tune the filter to provide a reasonable weight signal and eliminate higher frequency noises.
A recorder or oscilloscope was required to determine the point at which sampling of the net weight signal was taken. The window over which the weight signal was read was a potentiometer type adjustment requiring human intervention. Furthermore, the filter value was dependent on the package speed and weight and, therefore, any time a speed or weight change was required, manual intervention and resetup were required. Analog filter values also limited the rate at which packages could be weighed due to charge and discharge time of the filters being used.
The development of microprocessor controlled checkweighing has eliminated many of the difficulties inherent in the prior art systems discussed above but checkweighers are still designed to weigh every package for a fixed amount of time called the "read scale time". As the item to be weighed moves across the weigh pan, the weight curve rises, bounces, stabilizes for a period of time, and then falls as the package exits the weigh pan. During a portion of the period of time for which the package has settled, the checkweigher samples the weight curve and determines the weight of the package.
The period of time during which the package is stable and therefore "weighable" depends upon the length of the package, the speed of the conveyor belt, and the length of the weigh pan. Normally the speed of the belt is fixed and the length of the weighpan is always fixed. Therefore the length of the period of weighability is chiefly a function of package length. The shorter the package, the longer the period of "weighability". The longer the period of weighability, the longer the read scale time can be. Within the range of read scale times typically available to in-motion checkweighers, it has been established that the longer the read scale time, the better the accuracy. This is because a longer read scale time enables the system to filter out more of the low frequency noise which compromises checkweigher accuracy. Thus it is desirable to have as long a read scale time as possible.
Checkweighers use one of two strategies to overcome problems created by fixed read scale times. In one approach, the read scale time is fixed for all packages to the value permitted by the longest package to be weighed, i.e. a short read scale time. In such systems, the shorter package will not be weighed as accurately as they can be, or in some situations- as accurately as they need to be.
In the second approach, the different size packages are presented to the checkweigher in a slow and predictable manner, so that it can choose from a variety of different read scale times stored in its memory. This is too expensive. Packages have to be separated and sorted by length plus it is too expensive to slow down production for benefit of the checkweighing operation. Furthermore, it is complicated to arrive at a set of different read scale times from a series of different calibrations.
The potential errors and operator skill requirements of prior art systems are obvious and this has led to constantly increasing packaging costs and thus increase consumer costs.